[RISCV] Change the InstFormat for Zicbop prefetch instructions to InstFormatOther. (llvm#148934)

The lower 5-bits of the immediate are not part of the address unlike
other InstFormatS instructions.

We use InstFormatS in RISCVRegisterInfo::needsFrameBaseReg and
RISCVRegisterInfo::getFrameIndexInstrOffset which is not aware of this
special encoding. Force the format to InstFormatOther so those functions
will ignore it.

InstFormatS is also used by relocation emission, but I don't believe we
ever emit these instructions with a relocation because of the encoding.

Author: topperc

llvm/lib/Target/RISCV/RISCVInstrFormats.td

@@ -193,7 +193,9 @@ class RVInstCommon<dag outs, dag ins, string opcodestr, string argstr,
   let AsmString = opcodestr # !if(!empty(argstr), "", "\t" # argstr);
   let Pattern = pattern;
 
-  let TSFlags{4-0} = format.Value;
+  InstFormat Format = format;
+
+  let TSFlags{4-0} = Format.Value;
 
   // Defaults
   RISCVVConstraint RVVConstraint = NoConstraint;

llvm/lib/Target/RISCV/RISCVInstrInfoZicbo.td

@@ -41,6 +41,7 @@ class Prefetch_ri<bits<5> optype, string opcodestr>
               opcodestr, "${imm12}(${rs1})"> {
   let Inst{11-7} = 0b00000;
   let rs2 = optype;
+  let Format = InstFormatOther; // this does not follow the normal S format.
 }
 
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